The present invention relates to a method of manufacturing optical-use plastic products such as magnifying lenses, various finders, etc. for naked-eye viewing; spectacle lens and contact lens in ophthalmology; condensers; infrared ray projectors, radiation thermometers, solar furnace, vehicle mounted lighting devices in illumination and light-reception systems; various lenses in photography systems, interferometers, scanners, and in laser optical systems.
Generally, glass is conveniently used to manufacture optical products because of advantages including a rich choice of kinds, high stability of optical properties, high thermal resistance, and less change of properties due to temperature variation. However, the optical glass is not free from all defects. It is disadvantageously heavy and hard and needs a process of polishing, which leads to a reduced productivity. Also, a glass product having a non-spherical shape cannot be polished with a high reproducibility and thus, productivity is low. Furthermore, some kinds of the optical glass are poor in weatherability.
Accordingly, it has been proposed to use plastics for manufacturing optical-use products in order to overcome the above-mentioned drawbacks of optical glass. The optical-use plastic products are advantageous in that they are lightweight and have high impact resistance. At present, optical-use plastic is successfully used to make spectacle lenses. Other advantageous of optical-use plastics include ease variable forming, a reduced number of manufacturing steps by mass production using injection molding, and excellent reproducibility of product shape, etc.
The optical-use plastic products are manufactured by grinding and polishing as well as by injection molding. Recently, an injection and compression molding method, namely, an advanced injection molding method, has been proposed which includes (1) Rolinx method molding, (2) micro-molding, and (3) injection pressing. These methods will be further described herebelow.
(1) Rolinx method molding
A melted resin is injected into a mold held with a low clamping force. During the resin filling into the mold, the injecting pressure exceeds the molding clamping force to cause a parting line of the mold to open. Upon completion of the resin filling, the injection gate is closed and the mold is pressed to close the parting line. The mold, and thus the resin in it, are cooled with the mold kept pressed.
(2) Micro-molding
The parting line of the mold is closed with a mold clamping force. After a specified amount of resin is injected into the mold cavity, the molded resin is compressed by a separate independent pressurizing machine which is a hydraulic cylinder assembled in the mold or one assembled in a movable platen of an injection molder.
(3) Injection pressing
A melted resin is injected into the mold internal space which is increased for a compression stroke, the mold is closed in process or upon completion of the resin injection, and the molded resin is compressed by a clamping force.
These molding methods are used to provide plastic products having a dimensional accuracy that is increasingly higher than ever. For these methods, however, the injection molder and mold are designed to have a complicated structure and advanced precision molding techniques are necessary. In the technical and economical respects, these methods are not yet as popular as common methods in the industrial manufacture of optical-use plastic products.
However, the grinding and polishing method permits easy forming of flat and spherical surfaces with a high accuracy of working but cannot form non-spherical surfaces with high accuracy although it needs a great number of manufacturing steps. In the injection molding, as the temperature of a product removed from a mold falls the product will shrink, leading to a difference of product dimensions from those which the mold should assure. The phenomenon will take place more noticeably for products of 10 mm or more in maximum wall thickness and having an asymmetrical, unevenly-thick structure.
As mentioned above, the conventional injection molding is disadvantageous in that, as the temperature of a product removed from a mold falls, the product will shrink, leading to a difference of product dimensions from those which the mold should assure. Also, in the grinding and polishing method, a cast plastic block has to be machined to an intended shape (for example, lens shape) and then polished. This method can assure a high accuracy for flat and spherical surfaces but not for non-spherical surfaces and needs a large number of manufacturing steps.